As active materials to be used for positive electrodes of non-aqueous electrolyte secondary batteries, lithium-containing composite oxides have been widely used, and among them, a composite oxide containing cobalt has been in the mainstream. However, in a non-aqueous electrolyte secondary battery in a charged state where a battery voltage is raised from about 4.2 V (a positive electrode potential of about 4.25 V relative to metallic Li) to 4.45 V, a composite oxide containing cobalt phase-transfers from a hexagonal system to a monoclinic system. With the battery further charged, the composite oxide phase-transfers to the hexagonal system, but the monoclinic system appears again as the battery voltage reaches and surpasses about 4.6 V (cf. The Journal of Electrochemical Society, Vol. 141, 1994, P 2972 to 2977).
A monoclinic crystal structure appears due to distortion of a whole crystal. In a monoclinic composite oxide, therefore, binding power between oxygen ions which have a dominant role in maintaining a crystal structure and metallic ions which exist around the oxygen ions has decreased, and the thermal resistance of the composite oxide has significantly deteriorated. It has been known that a composite oxide may decompose when the thermal resistance thereof deteriorates, leading to generation of oxygen.
For the purpose of stably maintaining a crystal structure of a positive electrode active material even in an overcharged state, there has been proposed a technique of incorporating a specific element into a composite oxide (e.g. Japanese Laid-Open Patent Publication No. 2002-203553). It has also been reported that the similar technique allows improvement in cycle characteristic of a non-aqueous electrolyte secondary battery at a high temperature (e.g. Japanese Laid-Open Patent Publication No. 2001-319652).
On the other hand, there has also been proposed a technique of incorporating a specific element into a composite oxide containing nickel as a main constituent with the aim of stably maintaining the crystal structure thereof (e.g. Japanese Laid-Open Patent Publication No. 2002-289261).
It is however difficult to enhance stability of an active material only by controlling the composition thereof. Since a temperature for synthesis is described neither in Japanese Laid-Open Patent Publication No. 2002-203553 nor Japanese Laid-Open Patent Publication No. 2001-319652, for example, it is presumed that each of the active materials described in these publications is produced by, at least, baking a raw material mixture at below 1000° C., as conventionally done. This is because synthesis of an active material at a temperature of 1000° C. or higher usually causes occurrence of oxygen deficiency, or the like, in the production process, which undesirably lowers the stability of the active material. It is however considered that so long as the occurrence of oxygen deficiency or the like is prevented, the higher a baking temperature for raw materials for an active material, the higher structural stability of an active material can be obtained.
It is further mentioned in Japanese Laid-Open Patent Publication No. 2002-289261 that the disclosed composite oxide containing nickel as a main constituent has a pyrolysis peak in the range of 270 to 350° C. in a DSC measurement. However, the improvement in thermal stability of the active material described in Japanese Laid-Open Patent Publication No. 2002-289261 has limits and fails to reach a satisfactory level since the raw material mixture was baked at a temperature of 900° C. or lower in the production process of the active material. Furthermore, it is more difficult for the composite oxide containing nickel as a main constituent than the composite oxide containing cobalt as a main constituent to bring about an oxidation reaction for a change from bivalence to trivalence, thus easier to generate NiO to cause oxygen deficiency.